Download Electrochemical Cells

Electrochemical Cells
Electrochemical Cells
• Electrochemical cells are a way of storing
chemical potential energy.
• When batteries operate, electrons in high
energy states flow through wires, where their
energy can be harnessed to do useful work.
• For electrons to flow, there must be an
acceptor ion that will place the electrons in
lower energy states.
A voltaic cell spontaneously converts
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electrical energy to chemical energy
chemical energy to electrical energy
electrical energy to nuclear energy
nuclear energy to electrical energy
Electrochemical Cells
• In electrochemical cells, redox reactions are
separated into their individual oxidation and
reduction components.
• Each component is called a half-cell.
A Typical Galvanic Cell
• Zinc, the active metal, gives electrons to Cu2+,
the ion of the less active metal
Given the reaction that occurs in an
electrochemical cell:
Zn(s) + CuSO4(aq) → ZnSO4 (aq) + Cu(s)
• During this reaction, the oxidation number of Zn
changes from
• 0 to +2
• 0 to −2
• +2 to 0
• −2 to 0
The Anode
• The oxidation side (the side that loses
electrons) is called the anode.
• The anode is an electrode, a piece of an active
metal.
• The active metal loses mass when it is
oxidized; ions enter the electrolyte solution.
• The anode is the (-) terminal of the battery.
The Cathode
• The cathode is the place where reduction
takes place – electrons are gained by ions of
the metal in the cathode electrolyte.
• The cathode gains mass as a battery operates.
• When the cathode electrolyte is depleted, the
battery “dies.”
• The cathode is the (+) terminal of a battery.
• Anode – oxidation
an ox
• Cathode – reduction red cat
Which statement is true about
oxidation and reduction in an
electrochemical cell?
• Both occur at the anode.
• Both occur at the cathode.
• Oxidation occurs at the anode and reduction
occurs at the cathode.
• Oxidation occurs at the cathode and reduction
occurs at the anode.
Which half-reaction can occur at the
anode in a voltaic cell?
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Ni2+ + 2e- → Ni
Sn + 2e- → Sn2+
Zn → Zn2+ + 2eFe3+ → Fe2+ + e-
The Salt Bridge
• The salt bridge is essential for electrons to flow.
• The electrolyte solutions must be kept electrically
neutral.
• Every time an atom of an active metal is ionized,
an anion from the salt bridge enters the
electrolyte to balance off the new metal ion.
• Likewise, when a metal ion is reduced in the
cathode, a cation enters the cathode electrolyte
to restore the charge balance.
The purpose of a salt bridge in a
voltaic cell is to
• allow for the flow of molecules between the
solutions
• allow for the flow of ions between the
solutions
• prevent the flow of molecules between the
solutions
• prevent the flow of ions between the
solutions
The diagram represents a voltaic cell.
• Which statement correctly
describes the direction of flow for
the ions in this cell when the switch
is closed?
• Ions move through the salt bridge
from B to C, only.
• Ions move through the salt bridge
from C to B, only.
• Ions move through the salt bridge
in both directions.
• Ions do not move through the salt
bridge in either direction.
Given the statements:
A: The salt bridge prevents electrical contact between
solutions of half-cells.
B: The salt bridge prevents the direct mixing of one half-cell
solution with the other.
C: The salt bridge allows electrons to migrate from one halfcell to the other.
D: The salt bridge allows ions to migrate from one half-cell
to the other.
Which two statements explain the purpose of a salt bridge
used as part of a voltaic cell?
• A and C
• A and D
• C and D
• B and D
The diagram represents a voltaic cell.
• When the switch is
closed, which group
of letters correctly
represents the
direction of electron
flow?
• A→B→C→D
• A→F→E→D
• D→C→B→A
• D→F→E→A
Voltaic Cells and Table J
• The anode is the active metal
• The cathode is the less active
metal
• On table J, the anode is the
metal that is higher on table J
• Electrons flow from the anode
to the cathode.
Base your answer to the question on
the diagram of the voltaic cell.
Based on the given
equation, the balanced
half-reaction that
occurs in half-cell 1 is
• Pb(s) → Pb2+(aq) + 2e−
• 2Ag(s) → 2Ag+(aq) + 2e−
• Pb2+(aq) + 2e− → Pb(s)
• 2Ag+(aq) + 2e− → Ag(s)
Base your answer to the question on the diagram
of a voltaic cell and the balanced ionic equation.
• What is the total number of moles of
electrons needed to completely reduce 6.0
moles of Ni2+(aq) ions?
A student collects the materials and equipment
below to construct a voltaic cell.
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two 250-mL beakers
wire and a switch
one strip of magnesium
one strip of copper
125 mL of 0.20 M Mg(NO3)2(aq)
125 mL of 0.20 M Cu(NO3)2(aq)
Which additional item is required for the construction of
the voltaic cell?
• an anode
• a battery
• a cathode
• a salt bridge
Lab 20: Electrochemical Cells
Materials
Materials
1 M CuSO4, 1 M Zn(NO3)2, 1 M Mg(NO3)2, salt
solution
24-well plate Mg, Zn, Cu strips
scissor, filter paper
red and black electrical wires with alligator clips
voltmeter
Procedure
– Place a dropper full of zinc nitrate and copper sulfate
in consecutive wells.
– Attach a black lead to the black terminal of the
voltmeter; attach a red lead to the “5” red terminal.
– Take a piece of copper and attach it to the red
alligator clip
– Take a piece of zinc and attach it to the black alligator
clip
– Cut a small thin length of filter paper and saturate it
with salt solution.
– Place the “salt bridge”
between the two wells,
with one side in each well
– Place the copper electrode
in the copper solution, the
zinc electrode in the zinc
solution. Record the
voltage.
– Take the salt bridge out and see what happens to
the voltage before moving on to the next cell.
– Repeat the procedure for Zn/Mg and Mg/Cu cells.
Draw pictures of your cells on the following page.
– On your diagram, show which electrode is the
anode (oxidation electrode) and which is the
cathode (reduction electrode)
–Show the direction of the electron flow
(from anode to cathode)
–Write the oxidation half reaction.
–Write the reduction half reaction.
–Write the balanced equation for the
reaction that takes place.
Zn/Cu cell
_____ V
Ox ½ rxn
Red. ½ rxn
Balanced Equation
Zn/Mg cell
_____ V
Mg/Cu Cell
______V
Questions
• Which cell had the largest voltage? ____________________ Based
on the activity series, explain why.
• What is the function of the salt bridge?
• What happens to the voltage when the salt bridge is removed? Give
an explanation for this effect.
• Using Table J, explain how you can determine the anode and the
cathode of an electrochemical cell
A student constructed a cell using copper and silver
electrodes with copper (II) sulfate, and silver
nitrate as electrolytes. Sketch a diagram below
and label the following:
a) anode and cathode, with signs (+/-)
b) electrolytes in half cells, with ionic formulas
c) direction of electron flow through the wire
d) salt bridge with direction of (+) and (-) ion flow
e) In the electrochemical cell above, the copper
electrode loses mass when the cell operates.
In the Zn|Cu cell you constructed, the copper
electrode gains mass. Explain the difference
between these cells.
Electolysis
• Draw and label a picture of your electrolytic
cell. Identify the anode, cathode, show the
direction of electron flow.
• Explain why copper is both oxidized and
reduced in this reaction.
• Why are two identical electrodes used?
• Compare and contrast voltaic and electrolytic
cells (at least 4 similarities/differences)